Truing an out of round commutator 1

[a70duster]

Hello all,

I work for a ski resort and we are having trouble with one particular motor. We have discovered that the commutator is out of round and we believe that is giving us trouble. The first thought is to remove the LARGE motor with a crane from a remote location and deliver the motor to a rebuilding shop.

My idea is to turn the motor via a backup diesel and attach some type of cutting tool that is similar to a lathe. Anybody have any experience and/or ideas with this?

Thanks,

Mike

 

[aolalde]

For drag-lines we have a set of 4 generators moved by a sinchronous motor. We have turned the commutator outside diameter of the generators on site.
We adapt a small portable turret to the frame or end brackets and displace the cutting tool by handwheel operation. That should be a similar situation with your motor driven by an engine. You need to undercut the mica insulation between commutator bars too.

 

[UKpete]

It is also done (and has been for years) on one particularly troublesome dc traction motor here in the UK. I haven't seen it myself but I understand that the locomotive is placed on a wheel lathe to spin the motor by rotating the wheelset.

I would imagine that they take the brushes out and keep a vacuum cleaner on to keep fine swarf out of the motor.

 

[MikeHalloran]

It can work, but it's a waste of time if the bearings are not in good shape.



Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[edison123]

In situ machining requires good access to the entire commutator length, skilled machinist, custom built tool post, very low machining speed and skilled technician to undercut the mica.

 

[powerjunx]

machining of the commutator, while the machine is intact, is feasible.
a problem will exists on undercutting and chaffing of commutator bar segments which will require a good working access to the unit. where on chaffing works, you will maintain the 90 degrees segment edge separation. an experienced technician is required on this matter. whilst, a poor quality chaffing will decrease the length of the carbon brushes faster as expected.

 

[a70duster]

I appreciate all the replies


From aolalde

<< We have turned the commutator outside diameter of the generators on site.>>

Do you have any pictures of the setup? And do you have any information how one builds the cutting tool/portable turret?

Mike

 

[automatic2]

I have in the past performed similar tasks to machine cast clutch wheels of 70" diameter and larger. Your concern will be the rigidity of the tool post. I had used a machinist's vise to hold the tool yet provide micro traversing. The vise mount was custom made to fit the surrounding structure.

Carefull evalutation will be required as you will likely be 'reaching' with the tool which subjects it to chatter or worse.

 

[elechris]

Hi a70duster how are u,

I have completed this type of job many times on draglines, lift motors, winder motors and electric trucks (Komatsu).

1. How far out of round is the commutator...have u checked with a dial gauge to confirm your theory? If it is out of round you will notice excessive bruch wear also

2. Initial thought should be given to the length of the cutting tip to the tool post mount, if the distance is more than 6-8inches you could be playing with fire. There is nothing worse than a commutator that has been machined on-site.....and is worse when finished. Excessive vibration caused by a cutting tool can be a killer!!!

IDEA - You can run the DC motor via 2 welders....one will be connected to the "armature circut" and the other to the "field circut"...the field circut will be your speed control.
Ideal cutting speed will be between 100-400 rpm.

3. If it is successful you will then need to undercut and chamfer the commutator bars. This is not a job for an inexperienced person, it is critical that the angles of the chamfer are correct. If all has been successful make sure u check again with a dial gauge....for furture reference. *** Make sure u do not have any high MICA***

4. Don't be afraid to stone the commutator after all has been done.

Note: If the commutator has bands, give them a tap test and make sure thay are all solid. Sometimes loose bands can cause comms to lose shape!
Good Luck...hope some of this has been a help, if you want any other details or pics......show your email address and i will send some pics. Cheers, Chris (Australia)

 

[UKpete]

I have this thought in my mind that I have also heard of someone grinding a commutator in situ i.e. with a rotary grinding wheel. It could be nonsense; if it is true, I guess you would have to be careful in your choice of wheel.

 

[elechris]

Just a thought, if u are having trouble finding a "Commutator Stone".....follow this link. It is an Australian company and will give you an idea of the product to assist you.

http://www.morgancarbon.com.au/Maint products 2.pdf

Depending on the damage/severity you have on-site, this may get you out of trouble. This should only be seen as a short term solution, the machine may have to be removed to fix this problem...once again...good luck...chris

 

[motorman]

We use these. A little expensive for one time use.

http://www.martindaleco.com/html/CommutatorGrinders/CommutatorGrindingandTurningTools.htm

A comment about hooking up to a welder. Yes, it works but watch out for dangerous, blinding, flash over. Many years ago I watched a service engineer do this in a ship. Told myself at the time that I would never be that hungry for a job!

 

[GopelT]

The standard way of doing this is to obtain a grinding rig. Martindale or Ideal can supply this. You remove one of the brush rigs and make a fixture that bolts up to the brush rig support and that the grinding rig bolts to. This must be true to the shaft. Run the machine slow using the power supply for it if its adjustable and grind from coarse to fine. Slotting and beveling will have to be done and there are small handheld machines to do this also. Im curious though. what leads you to believe that the comm is out of round... what problems are you experiencing?
J.